1. Field of the Invention
The present invention relates to a method of and an apparatus for obtaining high-purity products by crystallization from liquid mixtures containing crystallizable components, and further relates to the foregoing method and apparatus which are particularly suitable for large-scale purification processes. More specifically, the present invention is suitable for large-scale and multi-stage purification of acrylic acid and methacrylic acid.
2. Description of the Prior Art
It is known, for example, that commercially produced acrylic acid usually contains such impurity components as acetic acid and propionic acid and the concentrations of these impurities are about 0.1% in total. Recently, usage of acrylic acid has been expanded, and in some cases, very high-purity acrylic acids are required. For example, the impurity concentrations in the order of some tens to hundreds of ppm are required for paper diapers.
In general, impurities are removed by distillation. However, it is very difficult to remove such impurities as acetic acid and propionic acid by distillation since these impurity components have boiling points close to that of acrylic acid. Under such circumstances, it has been proposed to remove these impurities by crystallization.
Two typical crystallization methods are available; one being that seed crystals are put into a liquid mixture containing a crystallizable component so as to nucleate and grow crystals in a suspended state in the liquid, and the other being crystals are formed and grown on cooled surfaces.
A mixing vessel type crystallizer is used for the former method. In this type of crystallizer, however, heat exchange area tends to be insufficient to produce a large quantity of crystals from liquid mixtures. Further, in case of adhesive crystals, such as, acrylic acid, cooling coils can not be used although it is necessary to scrape the crystals off the cooled surfaces. Therefore, the heat exchange area is definitely insufficient. Moreover, since solid-liquid separation is unavoidable, the structure and operations of this type of crystallizer are complicated to perform multi-stage crystallization.
The latter method is described in, such literatures as, "Handbook of Industrial Crystallization, Butterworth-Helnemann, 1993" edited by A. S. Myerson and "Introduction to Easy Practical Crystallization Process (KEMIKARU ENJINIARINGU), pages 76-83" September, 1992 by Masakuni MATSUOKA. According to these literatures, the latter method is advantageous in view of easy handling and of high crystallization rate. On the other hand, disadvantage is that the purity of the crystals is low since the impurities are trapped within the crystal layers in the course of crystallization. These literatures further indicated that the so-called sweating or partial melting of crystals is effective for removing the trapped impurities to improve the purity of the crystals.
U.S. Pat. No. Re. 32,241 which is the reissue of U.S. Pat. No. 3,621,664 discloses a multi-stage fractional crystallization technique for performing the latter method. In this crystallization technique, the crystals are grown on inner tube surfaces. However, this crystallization technique has a drawback that crystallization area, that is, heat exchange area decreases as crystallization proceeds since inner diameters of the crystal layers are reduced as the crystallization proceeds. Melting of crystals starts from the crystals contacting the tube inner surfaces. As a result, outer diameters of the crystal layers reduce and thus crystals are isolated from the tube inner surfaces so that the disclosed crystallization technique has a further drawback that the crystals tend to fall off the tube inner surfaces during the melting step. In this case, for example, metal screens or grids are provided at the bottoms of the tubes to prevent the crystals from falling off due to the clearance caused between the tube inner surfaces and the crystals. However, heat transfer rates are significantly lowered and extremely long time is required to melt the crystals. Although the recovered melt is heated and supplied into the tubes, the liquid flows through the space between the tube inner surface and the crystals and does not work to reduce melting time. These drawbacks are present even when the crystal layers are formed on the tube outer surfaces. Still further, in case of using a number of tubes in parallel, it is not easy to supply uniformly the liquid mixtures and the heat transfer mediums to those tubes and to form uniform films. Furthermore, it is costly to assemble a large number of tubes in parallel.
On the other hand, another type of crystallizer is known, which is similar to usual plate heat exchanger. In this type of crystallizer, however, fluid flowing passages are narrow and very complicated. In addition, both of the liquid mixture and the heat transfer mediums should flow filling up the fluid passages and should flow upward. Accordingly, considerably sufficient strength is required for the plates and the structures of the crystallizer. Further, crystallization rates are not high enough and purification is not so effective. Moreover, scale-up of the crystallizer is not easy from a practical point of view.